Any way to get the area of a mask to use in an expression?

I'd like to find the area of a mask, or even just the bounding width or height to use as a variable in an expression.

For example I'd like to have the brightness of a solid based on the area( or a dimension)  of a mask that resides in it.

I can't see that you can access these things, but I know that some scripts can, for example, access and change the individual nodes of a mask.

So there must be more to it than I have found, anyone know where to find info on modifying masks with expressions? perhaps that will lead me in the right direction.